Many manufacturing processes rely on the chemical or electrochemical reactions at the solid and fluid interface of a flow cell. Examples are electrochemical machining and solid catalysis reactions. Rates of chemical reactions between the fluid and solid interface are governed by the mass transport rate of the reactants or products at the interface. The mass transport rate, determined by Ficks' Law, is proportional to the concentration of the chemical species, the diffusion coefficient and is inversely proportional to the diffusion boundary layer thickness. As the diffusion boundary layer is determined by the fluid velocity near the solid surface, a higher flow rate increases the mass transport rate and, hence, the reaction rate. Therefore, agitation or circulation of the fluid is commonly used in chemical processes between fluids and surfaces to reduce the boundary layer and enhance the mass or energy transport rate.
As the volume of the fluid increases, strong agitation is required to circulate large volumes of fluid across the solid surface. Since the diffusion boundary layer is primarily determined by the flow velocity near the surface of the solid, the bulk of the fluid flow away from the solid and fluid interface has little effect on the diffusion boundary layer. A great percentage of the energy used for circulating the fluid is wasted. The need for a large flow rate also increases the complexity of the design of such chemical reactor systems. Furthermore, as a uniform diffusion boundary is critical for certain coating applications, such as electroplating processes, elaborate equipment design results in high capital cost.
Other means for increasing the mass or energy transport rate to facilitate reactions between fluids in contact with surfaces include vibration, impinging jet flow, or spray. However, such methods are often either not practical or impossible due to other restriction of particular processes.
For process consistency it is advantageous to provide a uniform mass or energy transport rate across the solid surface. Membranes are commonly used to separate the anode and cathode compartments in electroplating but these membranes either impede or accelerate ion transfer in the bulk fluid only, still leaving mass transfer at the anode or cathode up to the uncontrolled boundary layer if other means, described above, are not used. Indeed, the electroplating systems described in U.S. Pat. Nos. 5,096,550 and 6,126,798 fail to consider control of the fluid flow at this interface. Anode bags described in U.S. Pat. No. 5,616,246 also fail to consider this problem. Other references such as JP4052296 and EP471577 also fail to adequately consider control of the solid-liquid boundary layer.
There is a need to control fluid flow at the solid and fluid interface, and enhance the mass or energy transport rate at the solid interface without circulating a large volume of fluid.